Hi R.,
Here are a few tricks I used on my 3 year old son (he has been completely toilet trained for about 6 months).
First of all, get to know what they do just prior to going to the bathroom. My son would hide or even say that he was going to hide. This was a sure sign that he was about to fill his pants. As soon as he said that, I would say, "Let's try to go potty" and not make a big deal out of it.
Also, as soon as he got home from daycare and on the weekends, we had naked time. Of course, we had a few accidents (pee only), but it made it really hard for him to have an accident because he noticed it right away when he peed on the floor. Keep in mind, you have to be really dilligent and recognize the pre-potty behavior I mentioned before. We also took him to the potty just about every hour to have him try to go pee. If nothing happened right off, then we got off and tried again in an hour.
And the old stand by is candy. We kept a jar of M&Ms on the bathroom counter. Everytime he used the toilet, he would get an M&M (1 for pee, 4 for poop). We also used stickers. He had his choice between candy and stickers, and surprisingly, he frequently chose stickers over candy.
And finally, we did the pee-pee dance. Everytime he used the toilet in the beginning, mom and son came bursting out of the bathroom doing the pee-pee dance, whooping, and hollering ("YIPPEE, Carson used the potty!! Pee-pee, pee-pee dance, Carson used the potty). When he did something right, we made a big deal out of it. If he had an accident, we did not make a big deal out of it. We just said, "oops, that's an accident, let's clean it up" (and he would help me clean it up).
I hope you find these tips helpful.